Level shifters are frequently used for shifting the low and high levels of a digital signal output from one part of a system to different low and high levels required by another part of the same system.
FIG. 1 and FIG. 2 both show the concept of a level shifter receiving a digital input signal 2 with logic levels of, for example, a controller. The level shifter has an output 4 at which positive and negative drive signals of an appropriate level are output to a distributed load 6 which is shown in a simplified way in FIG. 2.
FIG. 3 shows the principle of operation of an output stage of a level shifter according to the prior art. Two switches S1 and S2 are connected in series between a first voltage supply providing a voltage VL according to a low level of a digital signal to be output from the level shifter and a second voltage supply providing a voltage VH according to a high level of the digital signal. An interconnection node between switches S1 and S2 is coupled via an output of the level shifter to a resistive-capacitive load represented by a resistor RLOAD and a capacitor CLOAD. Switches S1 and S2 are controlled by the digital signal input to the level shifter and only one of the switches is closed at a time.
Switches S1 and S2 are often implemented by MOSFET devices, but also by bipolar transistors or JFETs. Other electronically controllable switches may of course be used as well.
While e.g. MOS field effect transistors provide a very low ON resistance thus approaching an ideal switch, they function as current sources or current sinks during switching until the voltage on the output has risen high enough to render the voltage across the MOSFET quite small, at which point they behave like low-value resistors. Thus, when switching, there is a period during which a significant voltage drop across the active MOSFET device occurs, while a current is flowing through it. This results in power dissipation in the device.
One of the technical fields in which level shifters are required is the field of Liquid Crystal Displays (LCD). Level shifters must here transform the logic levels of the control signals of a timing controller providing, for example, a difference between low and high voltage level of less than 5 V into positive and negative drive signals of an appropriate level which depends on a particular LCD display and can reach several tens of volts. Present-generation LCDs using amorphous silicon gates (ASG), also called Gate-in-Panel (GIP) or Gate-On-Array (GOA) need drive voltages between about 20 V to 40 V for the high level and between about −5 V to −20 V for the low level resulting in a voltage difference from 25 V up to 60 V.
In an LCD application, the level shifter must drive a distributed resistive capacitive load as shown in FIG. 1 which may be simplified to the circuit shown in FIG. 2.
Because of the capacitive nature of the load connected to its output, power dissipation in the level shifter varies with the magnitude of the low and high levels of the drive voltage, the switching frequency, and the values of the resistance and the capacitance of the load. Especially, total system power dissipation depends on the voltage difference between the low and high levels of the drive voltage, whereas power dissipation in the level shifter depends on the external resistance and the characteristics of the non-ideal switches used in the level shifter. If the value of the external resistor in the load is large, the MOS field effect transistor approximates an ideal switch. However, with a small resistive load as usual in LCD displays, the MOS field effect transistor behaves as mentioned above as a current source or a current sink during a large part of the switching period. High power dissipation leads to an increased temperature. If the temperature exceeds the display manufacturer's internal design rules, additional thermal management is required which increases costs.
Furthermore, a lower temperature improves the reliability of the level shifter and allows a higher integration. Additionally, reduced power dissipation reduces the power needed to be generated by the bias functions of the LCD, i.e. by the power supply for the level shifter. Thus, also the power supply is improved concerning reliability and possibility of integration.
With level shifters having a plurality of outputs, a so-called “charge-sharing” solution is known. In systems using charge-sharing, panel display columns are first driven alternately at voltage levels above and below the required voltage level by using e.g. inverters. Or in other words, always two neighboring columns are driven as out-of-phase pairs, with a duty cycle of exactly 50%. In the charge-sharing period, dedicated switches are closed which interconnect all columns allowing current to flow from the columns with a voltage above the voltage level to the columns with a voltage below the voltage level.